(1) Field of the Invention
The present invention relates generally to a process for the removal of hydrogen sulfide from gases utilizing a polyvalent metal chelate in an aqueous alkaline scrubbing solution.
(2) Description of the Prior Art
Processes for the removal of hydrogen sulfide from gases utilizing a polyvalent metal chelate to oxidize the hydrogen sulfide to sulfur with electrolytic oxidation of the reduced polyvalent metal chelate back to the original valence state suffer from two basic problems: (1) The polyvalent metal chelate is unstable under the process conditions and tends to degrade and become ineffective and (2) the maintenance of pH at an optimum of about 7-9 in an aqueous alkaline scrubbing solution in which the hydrogen sulfide gas is absorbed. The latter usually requires the addition of an alkali to compensate for the reduction in pH which tends to occur as the result of the absorption of hydrogen sulfide (an acid gas).
In U.S. Pat. No. 4,436,711 and U.S. Pat. No. 4,436,714, a fuel cell is disclosed as useful to regenerate a polyvalent metal chelate utilized in an aqueous alkaline solution to remove hydrogen sulfide from a gaseous stream. The polyvalent metal chelate is changed from the lower valence state to the higher valence state by regeneration. In U.S. Pat. No. 4,443,423, and U.S. Pat. No. 4,443,424, a process is described in which a gaseous stream containing hydrogen sulfide is led directly to the anode of an electrochemical cell which functions as a contact zone for removal of hydrogen sulfide from the gaseous stream.
None of these references have as their objects and purposes the control of pH in the contact zone of the process at an optimum level for maximum efficiency of a process for removal of hydrogen sulfide from a gaseous stream utilizing a polyvalent metal chelate in an aqueous solution. In addition, none of the prior art recognizes the degradation effect that hydroxyl radicals have upon the polyvalent metal chelate.
To date the only method known to counteract the tendency toward a reduced pH by the absorption of the acid gas, hydrogen sulfide has been the use of buffering agents or the controlled addition of an alkali such as sodium hydroxide to maintain the pH at acceptable levels of between 7 and 9. At pH levels beyond 9 the polyvalent metal chelate is rendered unstable and at pH levels below 7, the aqueous alkaline solution containing a polyvalent metal chelate is able to absorb substantially less hydrogen sulfide from the gaseous stream since there is insufficient residual hydroxyl ion to neutralize the acidity introduced by the absorption of the hydrogen sulfide.